1. Field of the Invention
The present invention relates generally to a communications system for determining a state of a transmitter and, more particularly, to a communications system for providing an estimation of the number of radios tuned to each of a plurality of radio stations, where the system does not specifically identify a particular radio.
2. Discussion of the Related Art
Digital audio radio systems (DARS) that generate compressed digital audio signals to be transmitted by a digital audio transmission source and reproduced in a receiver associated with the DARS are known in the art. Audio signals to be broadcast by the DARS are generated in a broadcast studio and then converted to digital data. The digital data radio signals are then sent to an earth based ground transmission station to be transmitted to a plurality of receivers within a reception area, or to be transmitted to one or more satellites orbiting the earth in a geosynchronous orbit. The satellites then transmit the digital radio signals to a defined reception area over the Earth. U.S. Pat. No. 5,592,471 issued to Briskman, Jan. 7, 1997, discloses a digital radio system of this type.
For DARS that use satellite communications, the compressed digital data is sent to the earth based ground station for transmission to one or more satellites on a radio frequency xe2x80x9cuplinkxe2x80x9d carrier. The satellite receives the uplink signals from the ground station and then re-transmits the signals to a defined area on the earth""s surface where radio reception is desired. For example, the satellite can have a xe2x80x9cdownlinkxe2x80x9d beam pattern that covers the continental Untied States. The receiver receives the downlink signal, decompresses it, and converts it back to an analog signal for both stereo channels using a digital-to-analog converter (DAC) for subsequent amplification and listening through speakers.
It would be highly valuable if radio station personnel, marketing analysts, potential advertisers and the like could automatically and readily obtain information concerning the number of radios that are tuned to the several available radio stations that are broadcasting radio signals at different times of the day. In other words, specific information concerning how many listeners are listening to a particular radio station at any given time would be valuable for advertising purposes. Currently, no practical technique exists for automatically determining what station a radio receiver is tuned to. Marketing personnel typically assess this information by specifically asking consumers what stations they listen to through questionnaires, surveys and the like. Obviously, this technique is limited in its accuracy, and the number of radio listeners able to be surveyed. A much better and more accurate surveying technique would be provided if the individual radios themselves transmitted a unique coded frequency based on the particular station that the receiver was tuned to, that was received and deciphered by a receiver.
It is possible to equip each radio with a conventional transmitter that transmits its own unique signal to identify which station the radio is currently tuned to. The individual radios could be separately identified based on different coding schemes, such as frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA), known to those skilled in the art. For an FDMA implementation, each radio would require its own unique frequency spectrum to identify each individual radio and the station it is tuned to. However, because there would probably be millions of radios being monitored at a time, each having its own unique frequency spectrum, an identification system of this type would not be feasible because of the large amount of bandwidth required. Similarly for both CDMA and TDMA implementations, each radio would require its own unique code and time slot, respectively. As with FDMA, COMA and TDMA each require bandwidth in proportion to the number of communication links being transmitted. For the satellite DARS discussed above, bandwidth is at a premium. Because it is only necessary to provide an overall measurement of the average listenership per station, systems which identify the specific radio provide more information than is necessary for ratings purpose.
For a ratings service system to be feasible, it would need to identify the number of radios tuned to the particular radio stations in a more practical and cost effective manner than that discussed above. It is therefore an object of the present invention to provide a technique for determining a state of a transmitter without the need to provide each transmitter with its own unique frequency or code signal.
In accordance with the teachings of the present invention, a communications system is disclosed that automatically and continuously monitors a state of a group of transmitters. In one embodiment, the communications system monitors the number of radios tuned to each of a plurality of available radio stations. Each radio generates a coded signal depending on which radio station the radio is currently tuned to. The coded signal is applied to a frequency generator that generates a unique frequency signal for each code that is then transmitted by the radio. Each radio tuned to the sa me station would transmit the same unique frequency signal at substantially the same power level.
The frequency signals transmitted by all of the radios are received by a receiver, such as a satellite based receiver, that separates the signals by frequency. The power received for each different frequency signal is then measured to give a total power output for that frequency. An estimation of the number of transmissions for each different frequency signal is determined as the measured power level minus the expected receiver noise power, divided by the expected received power of a transmission from a single radio. Therefore, an estimation of the number of radios tuned to each radio station being monitored can be obtained.